The invention pertains to a tangential blower also known as crossflow fan or transverse flow blower.
Such a blower includes an impeller having blade edges extending parallel in relation to the axis of rotation, a deflector partially surrounding the impeller, a vortex former, and side members completely or partially surrounding the ends of the impeller.
Such tangential blowers are manufactured in large-scale production quantities for use in fan-forced heaters, air-conditioning systems, convectors, copiers, projectors, slide-in units for electric and electronic apparatus, and the like. They comprise a vortex former of simple construction, which has to provide formation of a continuous vortex flow; stabilization of the position of the vortex center; or deflection of the intake and outlet air of the blower.
Stable operating behaviour of a tangential blower within performance and noise values requires a stable position of the vortex center as well as a low loss of intake and outlet flow. In meeting these requirements the guiding effect of the vortex-former walls can play an important part. The reason for this is to be seen in that at a low to medium throttling of the tangential blower, the vortex center lies very close to the vortex former, so that on the suction side of the vortex former there are caused high speeds of the flow medium.
The tangential blower may produce a noise sounding like a siren which distinctly exceeds the usual noise. This siren sound is known to experts as the "peak blade frequency", the pitch of this sound can be calculated in accordance with the formula: EQU f=(n.multidot.z)/60 (Hz)
wherein n is the number of revolutions of the impeller per minute and z is the number of blades of the impeller.
The blades of the impeller moving past the vortex former responsible for causing this siren noise. These blades are at this point being subjected to an alternating stress at a small angle of rotation because the direction of flow is subjected to a rotation of about 180.degree. and because of the transition from the surge or pressure space to the suction space which is a considerable difference in pressure.
Proposals have also already been made for reducing or avoiding the siren sound which, however, either do not have the desired effect especially in the cae of long tangential blowers, or else, in the case of short tangential blowers, effect a reduction of the air intake or output.